FLAG-tag, or FLAG octapeptide, is a polypeptide protein tag that can be added to a protein using recombinant DNA technology. It can be used for affinity chromatography, then used to separate recombinant, overexpressed protein from wild-type protein expressed by the host organism. It can also be used in the isolation of protein complexes with multiple subunits.

A FLAG-tag can be used in many different assays that require recognition by an antibody. If there is no antibody against the studied protein, adding a FLAG-tag to this protein allows one to follow the protein with an antibody against the FLAG sequence. Examples are cellular localization studies by immunofluorescence or detection by SDS PAGE protein electrophoresis.

The peptide sequence of the FLAG-tag from the N-terminus to the C-terminus is: DYKDDDDK (1012 Da). It can be used in conjunction with other affinity tags, for example a polyhistidine tag (His-tag), HA-tag or Myc-tag. It can be fused to the C-terminus or the N-terminus of a protein. Some commercially available antibodies (e.g., M1/4E11) recognize the epitope only when it is present at the N-terminus. However, other available antibodies (e.g., M2) are position-insensitive.

CCL16, a chemokine poorly characterized at the
functional level. Human CCL16 is a member of the CC
family, and its gene maps to human chromosome 17q. In the
mouse, only a pseudogene has been identified to date.
CCL16 is a functional ligand for CCR1, CCR2, CCR5, and
CCR8. Recombinant CCL16 demonstrated chemotactic activity
on human monocytes and lymphocytes. Based on the ability
of human chemokines to exert activity on and bind to murine
receptors, the TSA mouse adenocarcinoma cell line was transfected
with human CCL16 cDNA and, in comparison with other
cytokines, was shown to be the faster inducer of systemic
immune response due to massive, prompt infiltration of leukocytes.